A plastic material injection molding machine comprises a mold having at least one cavity for producing plastic material articles by injecting molten plastic material into the cavity. For unloading the article from the cavity it is necessary to open the mold. For that reason conventional molds of such machines normally have two mold portions which, when adjoining each other, constitute the mold. At least one of the mold portions may be moved along a first path of movement and may, thus be brought into a distance from the other mold portion. In that condition the mold is open and a handling system, for example a gripper or the like may be moved into the gap between the two mold portions for grasping and removing the finished plastic material article. As may easily be understood, the sequences of movement, i.e. the opening and the closing movement, respectively, of the mold as well as the entering and the exiting movement of the handling system must be precisely coordinated in order to avoid collisions and damages on the mold and the handling system. However, in order to achieve a high productivity corresponding to a minimum cycle time of the machine, the sequence of movements must be effected as quickly as possible with the lowermost delay times.
U.S. Pat. No. 5,250,239 discloses a method and an apparatus for controlling the sequence of movements between a mold and a handling system. The handling system comprises a gripper which, however, makes no difference for the subsequent considerations. According to that patent specification the movable mold portion and the gripper are moved under the control of "go" signals. The "go" signals are generated by utilizing as a criterion the position and the velocity of the respective other element. By doing so, it is possible to generate the "go" signal for one of the two elements already at a moment in time when the respective other element has not yet completely left the trajectory of the first element. As a consequence, the entering movement of the gripper into the mold is initiated at a moment in time when the movable mold portion has not yet entirely left the gripper's path of movement. This may be made because the gripper must first move through a certain distance before it passes by the collision contour defined by the movable mold portion. Additionally, there are delay times and reaction times causing a certain loss of time before the gripper actually starts moving after having received the "go" signal. The resulting total period of time makes it, therefore possible to effect the movement of the two element in a time-overlapping relationship.
When using a method of the afore mentioned kind one can drastically reduce the cycle times of conventional injection molding machines which mostly depend on how long the mold must remain open for unloading the finished articles. However, the method does not necessarily result in the shortest possible cycle times. One has, namely, to take into account numerous further parameters, for example the velocity and the acceleration with which the elements may be moved, in particular the maximum values for velocity and acceleration that may be set without adding damages to the structure of these elements. U.S. Pat. No. 5,250,239 mentioned above is silent to that aspect.
German text book "Wirkungsfeld Spritzgie.beta.maschine", VDI Verlag 1995, p.p. 183 and 192-193 discloses an injection molding machine in which the movements effected by the mold and the handling system must be as fast as possible in order to shorten the cycle time of the entire process as much as possible. This means that the handling system, for example a gripper, must always be operated at the maximum possible acceleration. As a consequence, triangular velocity vs. time profiles are set within short distance (below 1,000 mm length).
However, as will be explained hereinafter, controlling the gripper to always operate at the maximum structurally possible acceleration does not necessarily mean that the sojourn time of the gripper within the open mold is in fact as short as possible. Moreover, a gripper operation under permanent maximum acceleration and deceleration, respectively, results in a maximum load alternation. This, in turn, means that there is a considerable load on the bearings of the machine which is highly disadvantageous in view of premature wear.
It is, therefore, an object underlying the present invention to provide a method and an apparatus of the kind mentioned at the outset such that the sojourn time of the handling system or gripper within the open mold is optimized to a minimum value.